I had only heard about the Postojna Cave before coming to Slovenia. But it turns out that there are many caves (of course) as well as many very interesting or famous cave sites. Right next to the Postojna cave, a 10 minute car ride away for instance is the Predjama Castle, which is world's largest castle built in a cave.
This castle was built for defensive purposes only, and such sits in the cave opening on a massive cliff face. The castle has some normal structures in the front, but the back of it is built into the cave. At the top, the castle can be exited to the cave tunnels, and eventually through a vertical shaft to the top of the cliff. We're told that this was used to supply the castle during sieges, as well as to continue offensive operations from the back exit.
There's also (apparently separate) cave under the castle, further down at the bottom of the cliff. We did not have time to visit that, but there are regular tours there as well. Would have been very interesting though! Under the cave opening for the lower cave, there's also a deep river valley, perhaps that's part of the lower cave.
The castle can be visited just by walking through, a self-guided tour. I particularly liked of course the back side and the cave rising further up. But another really nice touch was the rock balcony, a tunnel cut into the side the cave opening, leading to a high-up balcony where you can view the valley from the cliff face. That is, a window in the cliff, not on the castle walls.
On the tour we learned that the castle has also appeared in a number of movies, and in the filming of a Jackie Chan movie, Armour of God, Jackie fell and had a serious accident.
If you visit the Postojna cave (see here), you can buy a combination ticket that lets you visit Predjama castle as well, as well as a number of other interesting things. Much recommended. Our tickets cost 50 euros per adult, and included the Postojna tour and Predjama visit, as well as some exhibitions and the like in the Postonja area.
Cave behind and above the castle:
And these steps lead towards the top of the cliff exit inside the cave, eventually becoming a vertical shaft:
Not all rooms were as nice, however:
And the torture chamber was, well, torture:
The balcony on the cliff face, accessible from the castle: